edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 3 16 specimen that are far from the central probe position. Near interfaces this results in a decrease in accuracy of the strain measurement as the resulting diffraction patterns will contain contributions from both sides of the boundary. Another method for improving the precision of NBED disc position determination is precession electron diffraction (PED). In PED, the incident electron probe is tilted slightly off the optic axis and then azimuthally rotated (precessed) from 0° to 360° forming a cone of illumination. The diffraction pattern collection time is set to be equal to an integer multiple of the precession time. The effect of the precession process is to significantly dampen the effects of dynamic scattering. In turn, this results in a reduction of intensity variation in the diffraction discs and makes the determination of their location much more precise. While this method was not commonly available on most commercial microscopes in the past, this technique has seen widespread adoption in recent years and can be seamlessly integrated into the microscope acquisition interface. NBED combined with PED has been used to produce strain maps of Si/SiGe multilayer structures as well as from real semiconductor devices with very high precision (Fig. 3 panel i).[38] In the latter case, the strain field in a 22 nm transistor channel near recessed SiGe source and drains was mapped with precision on the order of 2 x 10-4. An alternative method for treatment of NBED data, which also seeks to overcome difficulties with disc position determination, has recently been developed and was inspired by cepstral analysis in audio signal processing. The so-called exit-wave power cepstral (EWPC) transformation[10] converts each reciprocal space diffraction pattern in an NBED dataset to a real-space pattern via Fourier transformation. This results in patterns with sharp peaks of uniform intensity as compared to the discs with heterogeneous contrast in the NBED patterns. The location Fig. 3 Examples of strain mapping. Panel i reproduced from Rouviere,[38] shows a strain analysis of a transistor device. (a) Virtual BF image of the transistor region showing the 22 nm channel (C) and the recessed SiGe source (S) and drain (D). (b)-(e) Strain maps calculated from N-PED patterns: εyy, εxx, εxy, and rotation (left to right). Panel ii reproduced from Ozdol,[40] NBED analysis of GaAs/GaAsP multilayer device cross-section. (a) HAADF image of the analyzed section. (b) Representative nanodiffraction pattern taken from the dataset with the [200] and [220] diffraction vectors that were analyzed for the strain analysis highlighted. (c) NBED strain map of εyy strain where y is the [002] direction. (d) NBED strain map of εxx strain where x is the [220] direction.
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